A common means of producing heat, especially large quantities of heat, such as required in furnaces, boilers, process equipment and so forth, is derived from combusting gas in air. Natural gas provides a highly desirable fuel for generation of heat for process applications since it is readily available and is not environmentally harmful, that is, the combustion of gas and air under ideal conditions provides relatively few harmful bi-products of combustion.
One problem that exists with gas burners of the type required to produce large quantities of heat is that such burners frequently produce substantial noise.
A common type of heating system utilizing gas includes the arrangement wherein a gas and air mixture is injected into a fire tube. A fire tube is typically a relatively long tubular member having air and gas injected at one end, the opposite end being connected to a stack for venting the products of combustion. The fire tube can be positioned in process equipment, such as an indirect heater in which the fire tube is placed in a water bath, a steam bath, on in a salt bath heater in which the products of combustion act as a heat transfer medium. A fire tube is one example of a type of enclosure in which burners are employed and with which the present invention is concerned.
In recent years increased attention has been given to reducing the noise from gas fired heaters. Many utilities, as well as distribution and transportation companies, and other industries in which heat is employed, such as the oil and gas industry, have experimented with ways of reducing the sound intensity of a gas fired heater. In densely populated areas of the United States, such as in the northeastern states, the effort to reduce the noise produced by gas fired heaters has attracted substantial attention. Some states have prescribed noise limits to be met by the users of gas fired burners. An example of the noise limitations that have been adopted include the restriction that the noise at the property line of a property on which a gas fired heater is employed cannot exceed 50 dbA. When it is realized that gas fired heaters are capable of producing sound intensities in excess of 100 dbA it can be understood that this sound limitation becomes a serious problem. As an example, in order to meet the 50 dbA at the property line, a heater located at a distance at 48 feet from the property line cannot produce a sound level exceeding 73 dbA measured at a distance of 3 feet from the heater. Reducing the sound that is generated by a typical gas fired heater that is, about 100 dbA, to 73 dbA has been a daunting task.
In order to reduce the sound intensities the type and arrangement of burners employed have been changed but substantial reduction in noise intensity has not been overly successful. It has been learned that there are basically three sources of noise originating from a gas fired heater. These are: (1) combustion process noise; (2) resonance or beat noise generated between the hot portion and cold portions of a gas fired heater, particularly of the type heater that uses a fire tube connected to an exhaust stack; and (3) secondary noise emitted from the heater shell, flame arrestor stack and other components connected directly to the heater. These sound sources fall across several octaves of the sound spectrum. The noise associated with the combustion process generally ranges from 400 Hz to 1000 Hz. The noise associated with the resonance of a fire tube type gas heater falls generally between 5 Hz and 30 Hz. The secondary noise source varies most widely and in a range approximately between 10 Hz and 300 Hz.
Techniques commonly employed to reduce the noise of gas burners, particularly of the type that use a flame arrestor, include the use of larger fire tube diameters and lower burner pressures. While these techniques result in reducing sound intensities, such sound reductions are achieved at the penalty of increased equipment costs. That is, a larger fire tube diameter is obviously much more expensive than a fire tube of a smaller diameter and, in addition, low pressure burners limit heater throttling to a narrow range sacrificing capacity and flexibility.
Others, in an attempt to reduce sound levels, have provided housings for burner assemblies or have used forced draft burners to allow higher pressure drops to be taken through silencers. Obviously, either of these efforts incur substantial increased costs.
A general object of the present invention is to provide a burner system that employs flame arrestors and critical placement of sound absorbing baffles and liners, to achieve reduction in sound levels without resorting to larger fire tube diameters, or resort to lower pressure burners and without the necessity, in most cases, of resorting to the use of housings or forced draft burners to overcome pressure drops associated with the use of silencers.
A better understanding of the invention will be obtained from the following description of the preferred embodiments, taken in conjunction with the attached drawings.